Electrolytic production of glassy layers on metals

ABSTRACT

METALS AND ALLOYS ARE COATED WITH GLASSY, ADHERENT, CORROSION-RESISTANT, PROTECTIVE LAYERS BY ELECTROLYTIC ANODIZATION AT HIGH VOLTAGE IN SUITABLE BATHS. THE METALS AND ALLOYS WHICH CAN BE COATED INCLUDE THOSE WHICH EXHIBIT ELECTROLYTIC RECTIFICATION AT BOTH LOW AND HIGH VOLTAGES AND THOSE WHICH EXHIBIT RECTIFICATION AT HIGH VOLTAGES ONLY. THE ANODIZATION BATHS CONTAIN ANIONS SUCH AS TUNGSTATE, PHOPHATE, ARSENATE, STANNATE, STIBNATE, MOLYBDATE, BORATE, CHROMATE AND DICHROMATE, ALONE OR IN COMBINATION AND CARBONATE, IN COMBINATION ONLY. SELECTION OF THE OPERATING CONDITIONS MAKES IT POSSIBLE TO PRODUCE COATINGS WHICH ARE SMOOTH OR WHICH HAVE EXTENDED SURFACES. THE PROCESS INVOLVES COAGULATION OF COLLOIDS AT THE SURFACE OF THE METALS. EITHER DIRECT OR ALTERNATING CURRENT MAY BE USED.

United States Patent Oflice 3,834,999 Patented Sept. 10, 1974 3,834,999 ELECTROLYTIC PRODUCTION OF GLASSY LAYERS ON METALS Rudolf .I. Hradcovsky and Otto R. Kozak, Long Beach, N.Y., assignors to Atlas Technology Corporation, Long Beach, N.Y. No Drawing. Filed Apr. 15, 1971, Ser. No. 134,431 Int. Cl. C23b 4/02, 11/02 US. Cl. 20456 R 23 Claims ABSTRACT OF THE DISCLOSURE Metals and alloys are coated with glassy, adherent, corrosion-resistant, protective layers by electrolytic anodization at high voltage in suitable baths. The metals and alloys which can be coated include those which exhibit electrolytic rectification at both low and high voltages and those which exhibit rectification at high voltages only. The anodization baths contain anions such as tungstate, phosphate, arsenate, stannate, stibnate, molybdate, borate, chromate and dichromate, alone or in combination and carbonate, in combination only. Selection of the operating conditions makes it possible to produce coatings which are smooth or which have extended surfaces. The process involves coagulation of colloids at the surface of the metals. Either direct or alternating current may be used.

BACKGROUND OF THE INVENTION The protection of metallic surfaces against corrosion by the process of anodization is at present limited to aluminum. Some protection is also provided by chromizing. However, none of these treatments, whether chemical or electrochemical, has proved to be sufiiciently effective against corrosive agents such as strong alkalis and strong acids. The anodized surface of aluminum is not resistant against acids or even mild alkalis. Also, such a surface is inadequate with respect to abrasion resistance. Fired ceramic coatings have been used to protect iron but the process incolves very high temperatures, so that application to most other metals is difiicult, if not impossible.

In many cases, resort has been had to organic coatings such as paints and enamels, applied by dipping, brushing or spraying. However, such coatings, although relatively resistant to non-oxidizing acids and alkalis, present other disadvantages including degradation at high temperature. Also, adhesion of the coating to the metallic substrate may be poor, particularly when temperature cycling is involved.

The deposition of a silica coating on aluminum has been described in Czechoslovakian Pat. No. 104927 issued Feb. 15, 1961 to Rudolf J. Hradcovsky. The deposition was carried out by making the specimen to be coated strongly anodic in a bath of aqueous sodium silicate or potassium silicate which might also contain a small quantity of ammonium molybdate. A coating with a low breakdown voltage was the objective, it being the intention to use the coated aluminum piece as an automatically introduced emergency by-pass in the event of a break in an electric circuit. As will become apparent, it is significant that no alkali was added to the silicate solution, so that the deposition was carried out in weakly alkaline solution.

It is evident, then, that the number of metals and alloys which can be protected by conventional processes against corrosion over a substantial range of temperature is severely limited.

SUMMARY OF THE INVENTION Generally speaking, in accordance with the invention, those metals which show an electrolytic rectifier effect when anodized electro-chemically can be coated with an adherent glassy layer. Examples of metals which show an electrolytic rectification effect at low as well as high voltages are Mg, Ta, Al, Ti, Nb, Ca, Zr, Hf, La, Mn, Ru, V and their alloys. These may be termed intrinsic rectifier metals. Metals which show this effect only at high voltage are Fe, Ni, Cr, Co and their alloys, and may be termed induced rectifier metals.

It should be noted that the term metal is used herein to denote both elementary metals and alloys.

Either type of metal can be coated with a glassy layer by immersing a specimen in a bath ranging from weakly acidic to strongly alkaline and containing at least one anion which yields a solid oxide on loss of charge. Suitable anions are tungstate, silicate, borate, phosphate, chromate, dichromate, arsenate, stannate and stibnate, alone or in combination. Carbonate is also useful in combination with other anions.

The deposition process is effected by making the specimen progressively more anodic until deposition of a colloid, and coagulation of the colloid on the surface of the specimen occur.

As the voltage is further increased above about 250 v. a discharge becomes evident at the surface of the metal. The process is continued under conditions such that the discharge is visible until the deposit reaches the desired thickness. A voltage of at least about 400 v. is necessary for the production of a satisfactory deposit. Layers of 1 mm. or more in thickness are readily prepared.

The process for coating metals of the second group is similar except that the voltage must be more rapidly increased during the anodization process and a minimum voltage of at least about 600 v. must be reached.

Accordingly, it is an object of the present invention to apply to metals which show an electrolytic rectifier effect a coating that is strongly resistant to corrosion and abra- SlOIl.

Another object of the invention is to provide for such metals a coating which serves as an adhesive underlayer for various burnt varnishes, paints, emulsions, and the like.

Yet another object of the invention is to provide for A further object of the invention is to provide for such metals a coating which ranges from very smooth to very rough and which can be made hard enough to abrade steel.

Still another object of the invention is to provide for such metals a coating which has a high voltage breakdown strength.

Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others thereof, which will be exemplified in the method hereinafter disclosed, and the scope of the invention will be indicated in the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To coat an electrolytic rectifier metal in accordance with the present invention, the metal is connected as the anode to a source of high voltage and immersed in a bath, the composition of which is given below. A second metal which is electrolytically insoluble is connected as the cathode to the source of high voltage and is also immersed in the solution. A suitable metal for such a purpose is iron.

3 Following are examples of suitable solutions:

Example 1: G. K siO 30 KOH 150 H O 1000 Example 2:

K PO 50 KI-ICO 50 KOH 2O K SiO 20 H O 1000 Example 3:

Nagwog NaOI-I 100 H O 1000 Example 4:

K130 50 KOH 30 B 1000 Example 5:

(NH4 5MD10z4-4H20 KOH 20 K SiO 20 H O 1000 Example 6:

SD02 Na SiO NaGH H O 1000 Example 7:

Sb O 15 K SiO 10 KOH 5 H O 1000 CONCENTRATION IN WEIGHT PERCENT 0F ANIONS OTHER THAN HYDROXYL Anion coneentration inweight Example Active anions percent 1 stow 1.25 2 POF3, H603, 5103- 5.50 3-. W042 5.05 4.- BOr 2. 4e 5.. MO7Oz 4HzO' Sim- I. SO 6-- SI103 ,si03 1.66 7 Shot-10 2.86

The strongly alkaline composition of Example 1 yields a hard layer of silica with a high breakdown voltage, in contrast to the type of coating achieved with a mildly alkaline solution as described in the Czechoslovakian patent cited above. The composition of Example 3 gives an extremely hard tungsten oxide layer.

For hardest layers, the baths should contain about 15% of alkali. Alkali concentrations between 0.1% and 15% are the most useful.

Other compounds which may be used for the same purpose when dissolved in water and brought to a pH ranging from weakly acidic to alkaline are the tungstate-silicate SiO -12WO '8KOH-10H O, the tungstate-borate H BO l2WO -5KOI-I- MH O, the tungstate-phosphate H PO 12WO KOH2 /2 H O, the tungstate-arsenate sodium tungstate, sodium silicate, sodium or potassium tetraborate and boric acid made alkaline with potassium or sodium phosphate.

Suitable combinations of compounds are ammonium molybdate with potassium tungstate and potassium borate with potassium tungstate. In general, the solution must contain at least one oxidic anion from which a glassy oxide can be formed by the removal of one or more oxy gen atoms. Thus, the borate and the silicate ions can yield B 0 and SiO in a glassy state. The carbonate ion, on

4 the other hand, while it has been found useful as a bath component, cannot form a glass by itself, since the prodnet of removal of an oxygen atom is CO a gas. As is evident, the various anions can be mixed with each other but, of course, the type of glass deposited will depend on the composition of the bath. Typical oxides which may be deposited, either alone or in combination, are AS203, AS205, B203, CI'203, C1303, M0203, S1102, 813203, Sb205, M002, M003, P203, P205, Slog, TIOZ, WO2, and W03.

The intrinsic rectifier metals include Mg, Ta, Ti, Nb, Ca, Al, Zr, Hf, La, Mn, Ru, and V, as well as their alloys. Examples of such alloys are Kovar, Rodar alloy 89446, Aluminum 3105, 3003, K845, Udimet 500, Mg AZ92A, A231B, A280A, Ti A, A, Ti6-A14V, and Ti-Pd Alloys.

Induced rectifier metals include Fe, Ni, Cr and Co as Well as their alloys. Examples of such alloys are the many different types of steels and the Fe-Ni-Cr combinations.

Certain compositions are particularly suitable for specific metals. For instance, a bath containing molybdate and phosphate ions produces glasses which are highly adherent to titanium and its alloys. The combination can withstand temperatures as high as 1200 C. without loss of adhesion; this is due to the fact that the coefiicient of thermal expansion of the glass can be matched to that of the metal.

Although all of the protective glassy layers have high breakdown voltages; particularly high breakdown voltages are obtained with layers containing tungstate, borate, molybdate, which have an average breakdown voltage of at least 800 v. at a thickness of 1 mm. as measured in accordance with ASTM test B-110-45. Tungstate when predominant in the glass, produces a highly homogeneous layer with a very smooth surface as can be seen by microscopic examination.

As further evidence of the protective action of layers deposited in accordance with the present invention, panels of Al 5667-H25 and 500-H14 were coated with a layer of silica 1 mm. thick. The panels were exposed to a spray of a 20% salt solution for 245 hours without developing any indication of corrosion. Similar panels have withstood the C.A.S.S. corrosion test for 48 hours, the Taber H-ll abrasion test and the 180 bond adhesion test.

To form the glassy layer on a metal substrate of the intrinsic rectifier type, the voltage is started at a low value and gradually increased. Initially, the substrate is oxidized, but when the voltage reaches about 250 v. coagulation of the solution begins. As the voltage is increased spark discharges appear on the surface of the anode. From the color of the spark discharges, it appears that the temperature of the spark discharges may be as high as 1500 C. It is believed that the high temperature resulting from the discharge melts the coagulated mixture into a homogeneous, pore-free glass on the surface of the metal. As indicated, the thickness of the layer depends on the current density and the duration of the process. The deposit may be smooth or rough, and the glass may be soft or hard, depending on its composition.

When the oxidation of the surface reaches maximum which is generally within a few seconds from initial immersion, the voltage must be increased in order to maintain flow of current and efficient deposition. Depending on the time and on the voltage used, glassy layers in excess of 1 mm. thick can readily be achieved. It should be noted that a high degree of reproducibility can be obtained by controlling the voltage, the current and the time of the deposition process. Also, the roughness of the surface of the glass layer can be minimized if the deposition current is maintained at a minimum value when the electrical discharge becomes evident on the surface of the electrode. The actual minimum current used will depend on the desired rate of deposition.

In coating intrinsic rectifier metals, the current density is held in the range of 250500 ma./dm. by adjustment of the voltage. For deposition of a satisfactory coating,

the voltage must be raised to at least about 400 v. and may go as high as about 1000 v. At the higher end of the voltage range the surface of the coating becomes rough and may be described as being extended. The coating, however, does not become porous.

The process for coating the induced rectifier metals is similar except that the voltage i raised rapidly to 250 v. and, in fact, this voltage may be imposed on the specimen prior to immersion in the bath. Also, the minimum voltage to which the piece must be raised to produce a satisfactory coating is about 600 v.

It should be noted that while there is no limitation on the size of specimens of intrinsic rectifier metals which can be coated by the present process, the diameter of specimens of the induced rectifier metals which can be coated is limited to about 2 mm. The length is not limited.

The color of the electrical discharge will vary in accordance with the cations of the bath. For example, sodium produces a yellow discharge, potassium a violet discharge, sodium tungstate a yellow-white discharge, and potassium tungstate K (H W O a white discharge.

In making up the bath, the presence of more than a small quantity of bromide, fluoride, iodide, chloride, sulphate, nitrate and cyanide ions is to be avoided because of the attack by these ions on the anode, creating conditions unsuitable for the coagulation of colloids on the anode.

The optimum bath temperature for coating is 45- 60C., but where the anode is large and substantial amounts of heat are generated during the deposition, it is desirable to use auxiliary cooling means.

The tungstate layer described above is particularly resistant to corrosion by strong alkali or strong acid environments. Also, a bath in which the borate ion is a domniant anion produces a glass with a high surface roughness. In fact, the roughness may be so great that the surface appears to be porous so that the layer may be more precisely described as having an extended surface. This type of glass has been tested for use as a sublayer for photosensitive gelatin emulsions in the printing field. It is also suitable as a sub-layer for paints and burned varinshes. Tests showed that such layers are of far greater durability than those presently used in this field.

It should be noted that alternating current, pulsing current or periodic reverse current can also be used for formation of glassy layers, since, due to the electroyltic rectifier effect of the substrate metals, current flows only when the metal is anodic.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efificiently attained and since certain changes may be made in the above compositions and methods without departing from the spirit and scope of the inven tion, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

What is claimed is:

1. Method of coating an electrolytic rectifier metal selected from the group consisting of Mg, Ta, Ti, Nb, Ca, Zr, Hf, La, Mn, Al, Ru, V, Fe, Ni, Cr, Co and alloys thereof with a glassy, adherent layer, comprising the steps of immersing said metal in a bath made strongly alkaline with an alkali metal hydroxide and containing at least one anion selected from the group consisting of B02, BO3 3, B4O7 -2, ASO4 3, CO CI'O4 2, 01107 6 MOO4 2, PO4 3, Ti3O'1 WO4 2, W7O246 and anions resulting from dissolution of SiO 12W0 8KOH -10H O, H BO 12WO SKOH' l4H O,

H3PO4' H20 and H AsO -9WO -3KOH-2 /zH O in water, CO being used only in combination with at least one anion of said group, the total concentration of anion other than hy droxyl lying between about 1.0 and 6.0 weight percent, causing current to flow by making said metal progressively more anodic until deposition, coagulation and visible spark discharge occurs at the surface of said metal, further raising the potential of said metal to a minimum of 400 v. for an intrinsic rectifier metal and to a minimum of 600 v. for an induced rectifier metal, and maintaining said flow of current until the deposit reaches the desired thickness, the diameter of induced rectifier metals being limited to about 2 mm.

2. Method as in claim 1, wherein said bath is maintained between 45 C. and 60 C.

3. Method as in claim 1, wherein said metal is an intrinsic rectifier metal and is selected from the group consisting of Mg, Ta, Ti, Nb, Ca, Zr, Hf, La, Al,- Mn. Ru, V and alloys thereof.

4. Method as in claim 1, wherein said metal is an induced rectifier metal and is selected from the group consisting of Fe, Ni, Cr, Co and alloys thereof.

5. Method as in claim 1, wherein the current density lies in the range of 250-500 ma./dm.

6. Method as in claim 1, wherein said metal is titanium and is coated from a bath containing molybdate and phosphate ions.

7. Method as in claim 1, wherein said metal is maintained anodic throughout the coating process.

8. Method as in claim 1, wherein said metal is alternately made anodic and cathodic.

9. The product resulting from the process of claim 1.

10. Method of coating an electrolytic rectifier metal selected from the group consisting of Mg, Ta, Ti, Nb, Ca, Zr, Hf, La, Mn, Al, Ru, V, Fe, Ni, Cr, Co and alloys thereof with a glassy adherent layer, comprising the steps of immersing said metal in a strongly alkaline bath containing SiO and alkali metal ion, causing current to flow, by making said metal progressively more anodic until deposition, coagulation and visible spark discharge occurs at the surface of said metal, further raising the potential of said metal to at least 400 v. for an intrinsic rectifier metal and to a minimum of 60 v. for an induced rectifier metal, and maintaining said flow of current until the deposit reaches the desired thickness, the diameter of induced rectifier metals being limited to about 2 mm.

11. Method as in claim 10, wherein said bath is maintained between 45 C. and 60 C.

12. Method as in claim 10, wherein said metal is an intrinsic rectifier metal and is selected from the group consisting of Mg, Ta, Ti, Nb, Ca, Zr, Hf, La, Al, Mn, Ru, V and alloys thereof.

13. Method as in claim 10, wherein said metal is an induced rectifier metal and is selected from the group consisting of Fe, Ni, Cr, Co and alloys thereof.

14. Method as in claim 10, wherein the current density lies in the range of 250-500 ma./dm.

15. Method as in claim 10, wherein said metal is maintained anodic throughout the coating process.

16. Method as in claim 10, wherein said metal is alternately made anodic and cathodic.

17. Method of coating an electrolytic rectifier metal selected from the group consisting of Mg, Ta, Ti, Nb, Ca, Zr, Hf, La, Mn, Al, Ru, V, Fe, Ni, Cr, Co and alloys thereof with a glassy, adherent layer, comprising the steps of immersing said metal in a bath made strongly alkaline with alkali metal hydroxide containing SO; and at least one anion selected from the group consistil'lg of BO'2 BO3 3, B4O7 2, ASO4 3, (303 CI'O4 2,

7 8 Cr O PO53, Ti O WO W and anions re- 21. Method as in claim 17, wherein the current density sulting from dissolution of SiO 12WO -8KOH-10H O, lies in the range of 250400 ma./dm.

H BO -12WO -K0H-14H O, 22. Method as in claim 17, wherein said metal is H PO "12WO -KOH-2 /2H O and maintained anodic throughout the coating process. H ASO -9WO -3KOH-2 /2H O in water, the total con- 5 23. Method as in claim 17, wherein said metal is alcentration of anion other than hydroxyl lying between ternately made anodic and cathodic.

about 1.0 and 6.0 Weight percent, and causing current to flow by making said metal progressively more anodic until deposition, coagulation and visible spark discharge occurs UNITED STATES E TS References Cited at the surface of said metal, further raising the potential of 1 said metal to a minimum of 400 v. for an intrinsic rec- 2778789 1/1957 204-56 M 3,293,158 12/1966 McNeill et al. 20456 R tifier metal and to a mimmumof 600 v. for an induced 2 346 658 4/1944 B a t 1 rectifier metal, and maintaining said flow of current unrenn n fi a til the deposit reaches the desired thickness, the diameter FOREIGN PATENTS of induced metal rectifier pieces being limited to about 15 397 453 11/1931 Great Britain 204 58 2 mm. t n

18. Method as in claim 17, wherein said bath is main- 342256 H1931. Great Britain 204-68 tained between 40 C. and 60 c. JOHN H. MACK. Prlmary Exammer 19. Method as in claim 17, wherein said metal is an L. ANDREWS, Assistant Examiner intrinsic rectifier metal and is selected from the group consisting of Mg, Ta, Ti, Nb, Ca, Zr, Hf, La, Al, Mn, 1 Ru, V and allo s thereof.

20. Method is in claim 17, wherein said metal is an 204-46 58 induced rectifier metal and is selected from the group consisting of Fe, Ni, Cr, Co and alloys thereof. 

